PROJECT SUMMARY Latent TGF? binding protein 4 (LTBP4) modifies muscular dystrophy in mice and in humans with Duchenne Muscular Dystrophy. LTBP4 is an extracellular matrix protein that binds latent TGF? to form the large latent complex. With stimulation like injury, LTBPs undergoes a conformational shift and proteolytic cleavage which releases latent TGF?, which is the first step in its activation. Activated TGF? has been implicated in fibrosis in many solid organs, and is a prominent feature of dystrophic muscle and injured muscle. Excessive TGF? is also known to inhibit muscle regeneration. In Duchenne Muscular Dystrophy, the protective LTBP4 variants that predict prolonged ambulation cause reduced TGF? signaling. Similarly, in mice the protective Ltbp4 allele is linked to reduced TGF? signaling. LTBP4 binds all three primary TGF? forms. Overexpression of the protective LTBP4 form in mouse muscle reduces fibrosis and also leads to increased muscle mass in the context of muscular dystrophy. LTBP4 not only binds TGF? but LTBP4 also binds myostatin. Domain mapping of LTBP4 binding sites demonstrated that TGF? and myostatin binding is mediated by distinct regions of LTBP4. LTBP4 therefore regulates multiple cytokines by sequestering these molecules in the matrix, rendering them inactive. A mechanism to reduce TGF? AND myostatin is an attractive target for treating muscular dystrophy since such an approach would reduce muscle fibrosis and promote muscle growth. This proposal outlines experiments designed to better define the mechanism by which LTBP4 acts. The first aim is designed to evaluate the relative contribution of TGF? and myostatin binding to LTBP4's effect. The second aim will investigate how manipulating LTBP4 affects fibrosis and muscle recovery after injury and in muscular dystrophy. The third aim will assess the importance of LTBP4's hinge region since this region mediates its effect as a modifier in muscular dystrophy.